The 17-year-old, who is also believed to be in Syria, told the Week in Week Out programme: "Jihad is obligatory."

By Frank GardnerBBC security correspondent

I have seen those pictures and I think that' is boasting, frankly.

Nasser Muthana is not going to come back to this country, unless he wants to be arrested, so I think that is him showing off on Twitter to his mates.

We have no idea if he even took the picture.

It is far more likely that whatever was in those containers, if there was anything in it, is going to be used in Syria against rival forces - either in Iraq or Syria.

Nasser and another man from Cardiff, Reyaad Khan, 20, appeared online two weeks ago with a man from Aberdeen in a video apparently filmed in Syria urging others to join Isis (Islamic State in Iraq and the Levant).

Now in a social media account, a man believed to be Nasser has posted a photograph which appears to show around 15 home-made bombs packed inside metal containers.

He also posted a photograph showing a home-made weapon said to be taken from an 'enemy' Shia Muslim.

The brothers' father Ahmed Muthana, 57, said this was "not the son I know", adding he wanted his children to come home even if they were imprisoned on their return.